The present invention relates to flame retardant additives to be used in conjunction with polyalkylene terephthalate molding resins. More particularly, the present invention relates to flame resistant polybutylene terephthalate molding resins having improved toughness containing, as a flame retardant, a high molecular weight halogenated epoxy resin and an antimony-containing flame retardant synergist.
Polybutylene terephthalate and reinforced polybutylene terephthalate molding resins have been found to be surprisingly superior to polyethylene terephthalate in many important processing and performance characteristics. For example, polybutylene terephthalate can be molded and otherwise processed at lower temperatures, have a shorter cycle time in the mold and do not require, as does polyethylene terephthalate, the presence of a nucleating agent to induce crystallinity. Furthermore, reinforced polybutylene terephthalate molding resins have correspondingly higher tensile strength, lower water absorption and better creep (flexural) properties than does similarly reinforced polyethylene terephthalate. As a direct result, these polybutylene terephthalate molding resins which were first disclosed, along with polyethylene terephthalate in U.S. Pat. No. 2,465,319 to Whinfield and Dickson, have been found to solve processing problems long associated with polyethylene terephthalate and believed, by those skilled in the art to be equally associated with all polyalkylene terephthalates Equally as significant, these molding resins present a noticeably improved balance of performance properties which those skilled in the art, did not believe to exist. Consequently, the superior processing requirements and physical properties of polybutylene terephthalate molding resins makes them more commercially desirable, with a wider area of applicability than polyethylene terephthalate.
These molding resins do, however, have one considerable drawback-they are flammable. Quite significantly, the presence of many important reinforcing agents, such as glass, enhances rather than deters the burning rate of these molding resins. Since the reinforcing agents have a direct effect on the desirable physical and mechanical properties of these molding resins, several commercially advantageous applications are precluded.
It is well known in general that halogen-substituted aromatic compounds are effective as flame retardants for thermoplastic resins, particularly those which are required to be molded at elevated temperatures. Thus, a variety of halogen-substituted aromatic compounds and polymers have been suggested as flame retardants for polyalkylene terephthalates, including polybutylene terephthalate. For example, tetrabromophthalic anhydride, tetrabromobisphenol ether, decabromobiphenol ether, hexabromobiphenol and brominated bisphenol-A as well as brominated polycarbonates formed from brominated bisphenol-A and phosgene have all been suggested as flame retardants for polybutylene terephthalate. Typically, the fire retardant package for the thermoplastic polyester molding compositions includes a halogenated aromatic fire-retardant such as listed above and a flame retardant synergist, most notably antimony trioxide. Brominated aromatic fire retardants for polyester resins including polybutylene terephthalate are disclosed in U.S. Pat. Nos. 3,873,491 and 3,936,400.
Recently, it has been suggested to incorporate a brominated epoxy resin as a flame retardant for polybutylene terephthalate molding compositions. A useful brominated epoxy resin is one formed by reacting tetrabromobisphenol-A and epichlorohydrin. This is exemplified in U.S. Pat. No. 3,965,212 wherein tetrabromobisphenol-A, optionally non-brominated bisphenol-A and epichlorohydrin are reacted to form a glycidyl ether-end capped brominated resin useful as a flame retardant for polybutylene terephthalate. A flame retardant synergist such as antimony trioxide is also added. The epoxy resins which are disclosed in U.S. Pat. No. 3,965,212, however, are of relatively low molecular weight having at most about 11 repeating units of the bisphenol-A-containing ether. Other patents which teach improving the flame resistance of polybutylene terephthalate with the brominated epoxy resins described above together with other brominated aromatic compounds include U.S. Pat. Nos. 3,909,485; 4,548,964; and 4,562,216.
In European Patent Application No. 0149190, published July 24, 1985, it is suggested to add a brominated aromatic flame retardant including tetrabromobisphenol-A bisdiglycidyl ethers to a co-polyetherester resin which comprises long-chain ester units derived from dicarboxylic acids and polyalkylene oxide glycols having a molecular weight of from about 400 to 6,000 and short-chain ester units derived from a dicarboxylic acid and low molecular weight diols. The brominated epoxies are described as having an epoxy equivalent of from about 1,700 to about 2,100 g/eq and a molecular weight of from about 2,000 to 30,000. Especially preferred are the F-2300 diglycidyl-type polymers having about 50% aromatic bound bromine as produced by Makhtashim Chemical Works, Ltd., Beer Sheba, Israel. The F-2300 flame retardant is described in the Journal of Fire Retardant Chemistry, Volume 9 (August, 1982), pp. 181-7. In this journal article, glass-filled polybutylene terephthalate is mixed with the F-2300 flame retardant brominated epoxy which is described as having a molecular weight of from 3,800 to 4,000. It is believed the F-2300 flame retardant is marketed by M & T Chemicals under the tradename Thermoguard 230.
Highly brominated bisphenol-A epoxy resins are also described in U.S. Pat. No. 4,221,893 assigned to Makhtashim. The brominated bisphenol-A epoxy resins disclosed therein are produced by reacting epichlorohydrin with the residue remaining after the formation of tetrabromobisphenol-A. These resins are described as flame retardant additives for polymers including thermosetting polyester resins.
It has been found that many of the aforementioned brominated flame retardants have the drawback that when the same are added to polybutylene terephthalate and the mixture subjected to molding, the resulting molded articles are greatly deteriorated in mechanical properties, particularly toughness. The brominated epoxy resins disclosed in U.S. Pat. No. 3,965,212, however, are described in the patent as imparting excellent mechanical properties to polybutylene terephthalate. Unfortunately, applicants have recently discovered that polybutylene terephthalate compositions containing the brominated epoxy resin disclosed in U.S. Pat. No. 3,965,212 show a marked decrease in melt flow relative to the polyester without the flame retardant. It is theorized that the low molecular weight brominated epoxy flame retardant is involved in intermolecular crosslinking with the polybutylene terephthalate.
In molding large articles or articles with complicated structures, it is important that the melt flow of the thermoplastic polyester be sufficient so as to allow the molten resin to uniformly fill the mold cavities without the need of substantially increased pressure and/or of modifying the molding equipment with consequent added expense to accommodate highly viscous molding compositions. Excessive molding pressure to accomodate highly viscous molding compositions may also lead to localized areas of frozen-in stress in the formed article since it is unlikely that the pressure will even itself out in a filled mold and, thus, excessive pressure is to be avoided. Obviously, the lower the viscosity of the plastic melt, the less will be the pressure needed to convey the molding resin into the mold fully and evenly. Accordingly, it is a distinct disadvantage that the incorporation of additives including the flame retardant additives sacrifice the melt flow of the molding resin composition.
Accordingly, it is a primary objective of the present invention to provide a flame resistant polybutylene terephthalate molding composition having improved toughness.
It is another object of the present invention to provide a flame resistant polybutylene terephthalate molding composition of improved melt flow characteristics.
It is still another object of the present invention to provide a brominated epoxy resin as a flame retardant which when added to polybutylene terephthalate molding compositions does not degrade the toughness of the molded article or disadvantageously affect the melt flow characteristics of the molding composition.
These and other objects and aspects of the present invention will be readily understood by those of ordinary skill in the art upon consideration of the following description of the invention together with the appended claims.